1. Field of the Invention
The present invention relates to a constant current circuit for supplying a constant current.
2. Description of the Related Art
At present, there is a case where a constant current circuit for supplying a constant current is mounted on a semiconductor device.
A conventional constant current circuit is described. FIG. 3 illustrates the conventional constant current circuit.
A K value (driving capacity) of a PMOS transistor P1 is higher than a K value of a PMOS transistor P2, or a K value of an NMOS transistor N2 is higher than a K value of an NMOS transistor N1. A gate-source voltage difference between the NMOS transistors N1 and N2 is generated across a resistor R1, and hence a current flowing into the resistor R1 is a constant current (see, for example, JP 2803291 B (FIG. 1)).
A conventional constant current circuit for low current consumption is described. FIG. 4 illustrates the conventional constant current circuit for low current consumption.
The K value of the PMOS transistor P1 is higher than the K value of the PMOS transistor P2, or the K value of the NMOS transistor N2 is higher than the K value of the NMOS transistor N1. When a resistor R2 is provided between a gate and source of the NMOS transistor N1, a gate voltage of the NMOS transistor N2 becomes lower and thus the NMOS transistor N2 operates in a sub-threshold region, whereby the current consumption of the constant current circuit reduces. A voltage obtained by subtracting a voltage generated across the resistor R2 from the gate-source voltage difference between the NMOS transistors N1 and N2 is generated across the resistor R1, and hence a current flowing into the resistor R1 is a constant current (see, for example, JP 06-152272 A (FIG. 1)).
However, the K values of the NMOS transistors N1 and N2 vary due to a fluctuation in gate oxide film thickness during a semiconductor device manufacturing process. Therefore, the gate-source voltage difference between the NMOS transistors N1 and N2 varies. Then, the voltage generated across the resistor R1 varies, and hence the constant current of the constant current circuit varies. In other words, the constant current of the constant current circuit varies due to manufacturing fluctuations in semiconductor devices.
The carrier mobility of a MOS transistor has a temperature coefficient. Therefore, when a temperature increases, the K value becomes lower. When a temperature reduces, the K value becomes higher. That is, when a temperature changes, the K value also changes. Thus, the gate-source voltage difference between the NMOS transistors N1 and N2 also changes. Then, the voltage generated across the resistor R1 changes, and hence the constant current of the constant current circuit also changes. In other words, the constant current of the constant current circuit changes with a change in temperature.
Therefore, a constant current circuit capable of supplying a stable constant current irrespective of the manufacturing fluctuations in semiconductor devices and the change in temperature is required.